Epicoccum nigrum

Epicoccum nigrum (1825) is a fungus with no known teleomorph form.[1] It has been classified as a member of the Hyphomycetes,[2] in the Deuteromycota, as well as the Fungi Imperfecti because it is only known to reproduce asexually. Despite that it is not yeast-like, it has been included in the broad, unrelated category of fungi known as black yeasts.[2] The fungus grows felty colonies in bright shades of yellow, orange, and red, often with brown or black throughout.[1][2] Colonies grow quickly, reaching about 6 cm in diameter in 2 days at room temperature.[1] Mycelia contain both chitin and cellulose.[1]

Epicoccum nigrum forms blastoconidia that are darkly coloured, warted and spherical, reaching 15 to 25 µm in diameter.[1] Conidia grow on a sporodochium, formed by warty and fibrous hyphae.[1][3] Sporets have been found to contain up to 15 cells.[4] The spores of E. nigrum are actively released depending on temperature, light, and relative humidity conditions.[5] The mechanism of release involves the separation of the conidium from the sporodochium via a double septum. It capitalizes on the spherical shape of the conidia, allowing it to "bounce" off the sporodochium.[6] Conidia then become airborne with movement or wind.[7] Sporulation is induced under Wood's light, or sometimes upon exposure to cold temperatures with a subsequent return to room temperature.[1] Pigment production is also sensitive to light and temperature changes.[8] Ideal growth temperatures range between 23–28 °C (73–82 °F), and ideal growth pH ranges from 5.0 to 6.0.[1] Although E. nigrum will grow in a range of water activity (aw of 0.99 to 0.97),[9] growth is optimized at water vapour saturation.[1]

Epicoccum nigrum produces a variety of biomedically and industrially useful metabolites, including important antifungal agents and pigments, including: flavipin, epicorazines A and B, epirodin, epicocconone, and a variety of carotenoid pigments.[10][11] Epicoccum nigrum has also been utilized in the biosynthetic manufacture of silver- and gold nanoparticles.[12][13]

A highly robust and ubiquitous fungus,[14] E. nigrum has an almost global spread, occurring in the Americas, Asia, and Europe.[1] Spores of E. nigrum have been cultured from a variety of environments, predominantly soil (i.e. peat, forest floor, raw humus, compost, tundra, sewage)[1] and sand (e.g., dunes, saline sands).[1][15] It is a saprophytic fungus, forming pustules (composed of sporodochia and conidia) on dead and dying plants.[4] This species is commonly found growing on cereals and seeds, as well as other crops including corn, beans, potatoes, peas and peaches.[1][16] It has been found to grow colonies on leaves submersed in water as cold as 0 °C (32 °F), and is considered a facultative marine fungus.[17] It is capable of colonizing algae and marsh grasses.[17] In indoor environments, E. nigrum has been found on paintings and wallpaper,[18] cotton and textiles,[1][4] in dust,[16][18] and in air.[1][16][19][20] It is tolerant of changes in water availability, and hyphal growth has been found to resume within an hour of exposure to water.[17]

Epicoccum nigrum has a wide array of medical, industrial, and agricultural applications. It produces a variety of pigmented and non-pigmented antifungal and antibacterial compounds.[11][21] These antimicrobial compounds are effective against other fungi and bacteria present in soil.[11] Flavipin, and epirodins A and B are pigmented antifungal agents;[10][11] non-pigmented compounds include epicorazines A and B.[11] Endophytic fungi such as E. nigrum are being explored as alternative sources of antibiotics to treat important resistant infections.[22] Polysaccharide antioxidants are also produced by E. nigrum.[23] Epicocconone is a fluorescent pigment unique to E. nigrum.[24] Epicocconone is valuable in terms of its ability to pigment cells orange, which then fluoresce red without impacting cell structure or function.[24]

Industrially, E. nigrum has a variety of broad applications. It has demonstrated a capacity to biosynthesize nanoparticles from silver and gold, which have applications in chemical, industrial, and medical processes.[12][13] It has been applied as biological treatment for mechanical oily effluent, reducing the content of hydrogen peroxide, phenols, and chemical oxygen demand in the oily effluent.[25] Epicoccum nigrum pigments have been considered as natural replacements for artificial pigments currently used in food.[26] It produces a variety of pigments, ranging from darker oranges to yellows and greens.[26] These pigments were synthesized by nonpathogenic strains of E. nigrum.[26]

In Brazil, E. nigrum is used to support root growth and control sugarcane pathogens.[27] It is a biocontrol antifungal agent active against brown rot in stone fruit, caused the species Monilinia laxa and Monilinia fructigena.[28] In contrast to these uses for E. nigrum metabolites, there has been an investigation into methods of controlling E. nigrum fungal colonies that have contaminated historic and cultural artifacts.[29] The fungus was found to be quite sensitive to essential oils from plants such as lavender and rosemary.[29] This is important in terms of the preservation of artifacts in humid climates, where fungal growth is an important determinant in the deterioration of stone structures and wood frames.[29]

Epicoccum nigrum produces the glycoprotein allergen Epi p 1 which binds to IgE, sometimes cross-reacting with other fungal allergens.[16] Cross-reactivity was found to exist with Alternaria alternata, Curvularia lunata, Cladosporium herbarum, and Penicillium citrinum.[30] Epicoccum nigrum is associated with respiratory fungal allergies, including allergic asthma, rhinitis, hypersensitivity pneumonitis, and allergic fungal sinusitis.[16][31] Two pediatric cases of hypersensitivity pneumonitis caused by E. nigrum were reported in children living in a damp and mouldy home, with daily exposure to E. nigrum in the shower.[32] The fungus has been found on human skin and in spit samples.[1] It does not typically cause systemic infection, although one case has been reported in an immunocompromised patient.[33]

Epicoccum nigrum has been treated under a variety of names in the genus Epicoccum. It was first identified in 1815 by botanist Johaan Heinrich Friedrich Link.[34] Today, all previously identified species are considered to be different variants of the species E. nigrum.[4] These include: E. purpurascens, E. diversisporum, E. versicolor, E. vulgare, E. granulatum, E. menispermi, and E. neglectum.[4][34] More recently, two distinct genotypes for E. nigrum have been identified with the combined use of DNA sequencing, morphology, physiology, and recombination factors.[35] This indicates the existence of cryptic species, and a subsequent call to re-classify E. nigrum into more than one species.[35]